First of all being drunk is not an excuse to do anything. If you do something you are responsible for that act. If you remember or don't remember you are responsible for that act. I am an addictions counselor so this is fact: Alcohol makes the inhibitors ineffective. These are what under normal circumstances keep you from acting like a fool. However, the intention and the desire to do something is there when you are sober but you are able to resist that desire. When drunk (or liquid courage) you now can act on your desire. He didn't lose his moral compass. He did what he wanted to do anyway
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He is the only one that knows that. He could have had a small attraction or he thought no one would find out. These reasons come to mind. Maybe he thought he would get away with it because he was drunk. It may have nothing to do with you (believe it or not). Ask him - he will probably blame the alcohol. You may not know that considering the circumstances.